Eye color Eye color is a polygenic phenotypic character and is determined by the amount and type of pigments in the eye's iris. Humans and other animals have many phenotypic variations in eye color, as blue, brown, gray, green, and others. These variations constitute phenotypic traits. The genetics of eye color are complicated, and color is determined by multiple genes. Some of the eye-color genes include EYCL1 (a green/blue eye-color gene located on chromosome 19), EYCL2 (a brown eye-color gene) and EYCL3 (a brown/blue eye-color gene located on chromosome 15). The once-held view that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. In human eyes, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris. The brightly colored eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids. Three main elements within the iris contribute to its color: the melanin content of the iris pigment epithelium, the melanin content within the iris stroma, and the cellular density of the iris stroma. In eyes of all colors, the iris pigment epithelium contains the black pigment, eumelanin. Color variations among different irides are typically attributed to the melanin content within the iris stroma. The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium. OCA2 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation.  Genetic determination of eye color See also: Human genetic clustering Eye colors can range from the most common color, brown, to the least common, green. Eye color is an inherited trait influenced by more than one gene. These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. These changes are known as single-nucleotide polymorphisms or SNPs. The actual number of genes that contribute to eye color is currently unknown, but there are a few likely candidates. A study in Rotterdam (2009) found that it was possible to predict the color of eyes with more than 90% accuracy for brown and blue, using just six SNPs (from six genes). The gene OCA2 (OMIM: 203200), when in a variant form, causes the pink eye color and hypopigmentation common in human albinism. (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within OCA2 are strongly associated with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an OCA2 regulatory sequence, where they may influence the expression of the gene product, which in turn affects pigmentation. A specific mutation within the HERC2 gene, a gene that regulates OCA2 expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are: SLC24A4 and TYR. Blue eyes with a brown spot, green eyes, and gray eyes are caused by an entirely different part of the genome. As Eiberg said: "The SNP rs12913832 [of the Herc2 gene] is found to be associated with the brown and blue eye color, but this single DNA variation cannot explain all the brown eye color variation from dark brown over hazel to blue eyes with brown spots."  Classification of color Iris color can provide a large amount of information about an individual, and a classification of various colors may be useful in documenting pathological changes or determining how a person may respond to various ocular pharmaceuticals. Various classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison. Others have attempted to set objective standards of color comparison. As the perception of color depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color. Eye colors range from the darkest shades of brown to the lightest tints of blue. To meet the need for standardized classification, at once simple yet detailed enough for research purposes, Seddon et al. developed a graded system based on the predominant iris color and the amount of brown or yellow pigment present. There are three pigment colors that determine, depending on their proportion, the outward appearance of the iris: brown, yellow, and blue. Green irises, for example, have blue and some yellow. Brown irises contain mostly brown. Eye color in animals other than Homo sapiens are differently regulated. For example, instead of blue as in humans, autosomal recessive eye color in the skink species: Corucia zebrata is black, and the autosomal dominant color is yellow- green.  Changes in eye color In all populations, children are most commonly born with unpigmented eyes. However, most babies who have European ancestry have light-colored eyes before the age of one. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continually produce pigment, in theory eye color can be changed. Most eye changes happen when the infant is around one year old, although it can happen up to three years of age. Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages. An iris that appears golden contains some melanin even at this early age and is likely to turn green or brown as the infant ages. Changes (lightening or darkening) of eye colors during puberty, early childhood, pregnancy, and sometimes after serious trauma (like heterochromia) do represent cause for plausible argument to state that some eyes can or do change, based on chemical reactions and hormonal changes within the body. Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined. Most eye-color changes have been observed or reported in the Caucasian population with hazel eyes.  Eye color chart (Martin–Schultz scale) Carleton Coon created this chart by the Martin–Schultz scale often used in physical anthropology. I. Light eyes Eyes light and light mixed are 16–12 in Martin scale. Light Gray, blue, green. Light-mixed a. Very light-mixed (blue with gray or green or green with gray) b. Light-mixed (light or very light-mixed with small admixture of brown pigment) II. Mixed eyes Mixed 12–6 in Martin scale. Mixture of light eyes (blue, gray or green) with brown pigment when light and brown pigment are the same level. III. Dark eyes Dark-mixed 6–4 in Martin scale. Brown with small admixture of light pigment. Dark 4–1 in Martin scale. Brown (light brown and dark brown) and very dark brown (almost black).  Amber Amber eyes / Golden green-brown Amber eyes in sunlight - displaying an orange color rather than brown Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This might be due to the deposition of the yellow pigment called lipochrome in the iris (which is also found in green eyes). Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, while amber eyes are of a solid gold hue. Even though amber is considered to be like gold, some people have russet or copper colored amber eyes which many people mistake for hazel, though hazel tends to be duller and contains green with red/gold flecks, like mentioned above. Amber eyes may also contain amounts of very light gold-ish gray, found in animals like wolves. The eyes of some pigeons contain yellow fluorescing pigments known as pteridines. The bright yellow eyes of the Great Horned Owl are thought to be due to the presence of the pteridine pigment xanthopterin within certain chromatophores (called xanthophores) located in the iris stroma. In humans, yellowish specks or patches are thought to be due to the pigment lipofuscin, also known as lipochrome. Many animals such as canines, domestic cats, owls, eagles, pigeons and fish have amber eyes as a common color, whereas in humans this color occurs less frequently, more in places like Brazil and Asia, being rare in other regions.  Blue A blue iris Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium, and shorter wavelengths are reflected and undergo Rayleigh scattering. The type of melanin present is eumelanin. The outer surface of the iris of a blue-eyed person is clear, lacking the outer layer of pigmentation that is found in brown eyes. Their color is caused by the inner layer of pigmentation and the semi-opaque fibrous tissues that lie between the two layers.  Distribution Blue eyes are most common in the Baltic Sea region, Northern, Eastern and Central Europe, and to a lesser degree in Southern Europe and southern Central Asia; Afghanistan and Pakistan are a notable example. Blue eyes are found in the Levant and the Middle East, especially amongst the Jewish population of Israel. However, many modern Israeli Jews are of European Ashkenazi origin, among whom this trait is common (53.7% of Ukrainian Jews have blue eyes ). While blue eyes are thought to be exclusive to Caucasoid ethnic groups, the manifestation of blue eyes has been documented in pure-blooded, darkly complected tribal Africans, as well as people of mixed African and European ancestry; the former, usually the result of genetic mutation and the latter most often the manifestation of recessive European genes. A 2009 study suggested that blue eyes were present in Siberia during the Bronze Age; 15 of 25 Andronovo culture specimens (60%) from the Krasnoyarsk area had blue (or green) eyes. Y-Chromosome DNA testing performed on ancient Scythian skeletons dating to the Bronze and Iron Ages in the Siberian Krasnoyarsk region found that all but one of 11 subjects carried Y-DNA R1a1, with blue or green eye color and light hair common, suggesting mostly European origin of that particular population. In 2005, a 2,500-year- old mummy of a Scythian warrior found in the the Altai, Mongolia, showed a 30- to 40- year-old man with blue eyes and blond hair. A 2002 study found that the prevalence of blue eye color among Caucasians in the United States to be 33.8 percent for those born from 1936 through 1951 compared with 57.4 percent for those born from 1899 through 1905. Blue eyes are continuing to become less common among American children, with only one out of every six or 16.6% of the total population, and 22.3% of the White population having blue eyes.
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